The present invention relates to a boiler for heating liquids, and more particularly, a boiler which is adapted to be fitted into a fireplace cavity and combined with a heating system.
In the past, the use of heat generated in a fireplace for heating a liquid which is circulated through a domestic heating system has not met with success. In one prior art device dealing with this subject, illustrated in U.S. Pat. No. 3,958,755 to Cleer, Jr., a water jacket is used which forms the interior walls of a fireplace. The use of a water jacket in a fireplace presents a possible safety hazard. A water jacket possesses large surface areas in contact with the water which renders the jacket inherently weak as a container. As steam pressure builds up within the jacket due to the boiling of the liquid therein by heat from a fire, the walls of the jacket tend to balloon to increase the volume within the jacket. It is apparent that a malfunction of the circulator of the system in which the water jacket is incorporated while a fire is burning in the fireplace could result in the buildup of such pressure in the water jacket that the walls of the jacket would burst and possibly injure persons nearby.
A further problem was encountered with such systems in the past in the control of circulation of liquid in the system in response to liquid temperature. The previously mentioned Cleer, Jr., patent discloses that control of water flow into the water jacket might be responsive to a temperature sensor within the water jacket. However, a temperature sensor which senses liquid temperature at only one point in the water jacket does not sense the average temperature of the water in the jacket, which can lead to periods of instability in circulator operation.
For example, when a fire is first built in the fireplace, the circulator is not operating and the water is stationary in the water jacket. The water is heated up to a predetermined temperature relatively quickly and the circulator is actuated. At this point the fire has not built up to a sufficient size to heat a continuously flowing volume of water to the predetermined temperature and so the circulator is deactuated as soon as the temperature sensor senses the incoming cold water. Again, the stationary water is heated relatively quickly and the circulator is actuated until more cold water is sensed.
This intermittent operation of the circulator continues until the fire has built up to a sufficient size to heat the incoming cold water to at least the predetermined temperature without the need for stopping water flow through the jacket. This period of instability also exists and is of greater duration when the fire dies out due to the lesser rate of temperature decrease in the fireplace than the rate of increase in temperature when the fire is first built. The cost of using a plurality of temperature sensors to determine the average temperature in the water jacket would be excessive.
A further drawback in the use of a water jacket is encountered during installation of the jacket. The water jacket must be designed to fit precisely within the dimensions of the fireplace into which it is to be incorporated. Due to the variations in size of fireplaces, it is not possible to manufacture a water jacket of standard size which is adapted for use with a plurality of different fireplaces. The need for customizing the water jacket to fit a particular fireplace presents an undesirable expense. Furthermore, once fitted and installed, the water jacket is not easily removed from a fireplace should the need or desire to do so arise.